Residential wood stoves are essentially closed, semi-sealed boxes where wood is burned. The wood is in large chunks which burn only on the surface thereof. However, the entire wood chunk becomes heated, leading to fractional distillation of organic compounds from the wood chunk interior. The organic compounds are released into a combustion chamber of the stove where the wood is located. The organic compounds are not completely burned and are discharged as air pollutants from a chimney connected to the stove.
Air enters the stove through a controlled opening, while smoke and combustion products leave through a second, uncontrolled opening and flow to the chimney. The burn rate of wood fuel is regulated by controlling the rate air enters the stove. The domestic wood stove is frequently operated in an air-choked mode at low burn rates, in the range of 1 kilogram per hour, resulting in high particulate, carbon monoxide and hydrocarbon emissions. At such low burn rates, the wood temperature is too low to ignite and burn the particulate, carbon monoxide and hydrocarbon emissions, causing these products of combustion to add to pollution.
Hence, a serious problem with wood stoves as domestic heating sources is the pollutants produced thereby as a result of incomplete combustion of the burning wood. The incomplete combustion causes excessively high particulate, carbon monoxide and hydrocarbon emissions.
In one prior art single chamber wood stove having primary and secondary combustion zones in direct communication with each other, i.e., where no baffle or wall is between the primary and secondary combustion zones, particulate emissions were measured at 25.4 grams per hour, carbon monoxide emissions at 126.3 grams per hour, and hydrocarbon emissions of 17.7 grams per hour. These data were collected while burning seasoned oak cordwood, with airflow settings from outside the wood stove to the primary and secondary combustion chambers set at minimum values therefor.
We have found through measurements that wood stoves having separate primary and secondary combustion chambers, i.e., chambers separated from each other by a baffle or wall, wherein wood is burned in the primary chamber, do not resolve the incomplete combustion problem or are very inefficient. Gases flowing from the primary combustion chamber to the secondary combustion chamber are cooled to such an extent that the particulates, carbon monoxide and hydrocarbons are not burned in the secondary combustion chamber. Measurements we have conducted on the commonly assigned U.S. Pat. No. 5,007,404, wherein gases in the secondary combustion chamber are ignited, have demonstrated that high particulate, carbon monoxide, and hydrocarbon emissions are still present.
A wood stove including separate primary and secondary combustion chambers, arranged so that the secondary combustion chamber is supplied with a hydrocarbon fuel (e.g., methane, propane or butane) from an external source is reported on pages 40, 49 and 51 of EPA Report 600/7-81-091. The gaseous hydrocarbon fuel is stated to be ignited by an afterburner in the secondary combustion chamber. Flue gas in the secondary combustion chamber is reported as having sufficient air to burn the fuel and combustible emissions in the secondary combustion chamber.
While this prior art arrangement produces a significant reduction in hydrocarbon emissions, the prior art two chamber stove has been basically converted from a wood stove to a gas furnace. This is because the flow rate of the gaseous hydrocarbon fuel is reported as being from 2 to 3 cubic feet per minute. The 2 to 3 cubic feet per minute flow rates are comparable to the flow rates of domestic natural gas furnaces. Hence, the device and method of operation disclosed in this prior art report are not satisfactory for actual domestic applications, wherein wood stove owners are attempting to minimize expenses and the use of fuel sources other than wood.
It is, accordingly, an object of the present invention to provide a new and improved efficient wood stove having low particulate, carbon monoxide and hydrocarbon emissions.
Another object of the invention is to provide a new and improved single chamber wood stove having relatively low particulate, carbon monoxide and hydrocarbon emissions by providing almost complete combustion of gases released from the burning wood.